Polychlorobiphenol-base products have heretofore been widely used in the field of insulation of electrical equipment, for, aside from their electrical properties, they combine a number of advantages making them very well suited for use as dielectric liquids: very great stability at high temperature and under hydrolysis, little or no flammability, low vapor pressure and moderate price. On the other hand, their absence of biodegradability leads to their accumulation in the environment, which considerably restricts their range of application and has prompted some countries to prohibit them totally or partially. Furthermore, at very low temperature, their dielectric properties drop rapidly, making them difficult to use under conditions of extreme cold.
Other products have also been proposed as dielectric liquids, e.g., the esters described in French Pat. No. 2,322,435, but those esters are readily combustible. As for the polychloropolyphenylalkanes described in French Pat. No. 2,273,351, they are made from alkane dihalides, e.g., dichloro-1-1-ethane, which on the economic level offers no advantage.